Turbofan aircraft engines have large fans at the forward end. These rotate at a high speed of about 4000 revolutions per minute.
Foreign objects such as birds, hailstones or debris ingested from the ground can occasionally strike the fan blades. It is possible for this to cause the fan blades to fragment with fragments on the order of 7 kilograms traveling at 930 meters per second. It is essential to contain these blade fragments without total destruction of the surrounding casing.
A containment structure of isogrid construction is shown in copending patent application entitled "Aircraft Fan Containment Structure" by Kulak, Dembeck and White. The inner skin of the isogrid's structure as taught there uses differing thicknesses so that proper resistance to blade penetration and rubbing is achieved with a minimum of excess weight. It is also suggested that the change in thickness of the skin not be too abrupt and that there accordingly be interface segments of intermediate thicknesses.
In accordance with the usual design and manufacturer of such a structure, the bulk of each triangular segment, of which there are over 1000, is machined out with the first tool leaving a skin thickness of the specified amount. A second tool is used to machine away the material under the flange of the flanged ribs around the triangular periphery. Conventionally all flanges are specified with the same thickness and accordingly when the skin thickness changes a different tool must be used to mill away the material under the flange.
The manufacturing procedure involves placing the particular tool in the milling machine and milling the isogrid segments of the entire case which used that particular dimension. The approximately 3 meter diameter case is fully rotated to achieve the access to all the appropriate segments the tool must then be changed and the apparatus repositioned for the next tool.
This requirement for multiple tools has been shown to incur considerable expense and take a lot of time. Accordingly it would be desirable to have a modified design which could be manufactured with fewer different milling tools without substantially increasing the weight.